Alder reaction of cyclopropene with butadiene

Diels-Alder Reaction of Cyclopropene with Butadiene 

Wiberg reported the Diels-Alder reaction of butadiene and cyclopropene [53] and Baldwin estimated from the reaction between cyclopropene and 1-deuteriobutadiene at 0°C that 99.4% of the formed cycloadduct was the endo isomer [54], There are many suggestions which attempt to explain endo selectivity in Diels-Alder reactions (Alder s rule [55]), but none are firmly established. According to Woodward and Hoffmann [56], the preference is the result of favorable Secondary Orbital Interactions (SOI) or secondary orbital overlap [57-59] between the diene and dienophile in the corresponding transition state structure. One can also find an explanation for the reaction preference in the difference between primary overlap [60], volumes of activation [61], and the polarity of the transition states [62]. Secondary orbital overlap between the diene and the dienophile does not lead to bonds in the adduct, but primary orbital overlaps do. 

There is no doubt that the driving force for cyclopropene as the dienophile for a Diels-Alder reaction is the release of angle strain energy in the course of the reaction. This is demonstrated by its relatively low activation barrier. For example, cyclopropene reacts with cyclopentadiene and butadiene at 0°C or at room temperature, producing almost exclusively the endo cycloadduct [53]. This addition can be explored by computing activation barriers for two isomeric transition state structures. In this way, nonbonding interactions between diene and dienophile in two isomeric transition state structures can be closely evaluated. The reactivity and selectivity for two concurrent reaction pathways can also be computed. 

One way of determining nonbonding interactions between two chemical systems is by computing bond orders [67-68], as well as by Frontier Molecular Orbital (FMO) [69-71] interactions in the transition state. It is well known that FMO can be used to explain the reactivity of a diene and dienophile for cycloaddition reactions [72-74]. There was no noteworthy difference between the 

ZPEC = zero point energy correction A exo 3 d - activation barrier for 

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